Selection of differential pressure gas flowmeter

Metering Devices and Applications Selection of Differential Pressure Gas Flowmeters Hu Haiyan (Metering Station of Technical Inspection Station of Hekou Oil Production Plant, Shengli Oilfield, Shandong Hekou Differential Pressure Gas Flowmeter is the most widely used gas flowmeter, and its usage ranks first among all kinds of flow meters. This article takes the channel flowmeter, V-cone flowmeter, and swirl flowmeter as examples to introduce how to select the differential pressure gas flowmeter and the issues that should be paid attention to in the selection.

Differential pressure gas flowmeter; selection; instrument performance Natural gas is transported by pressurized pipelines, and its flow rate refers to a certain cross section of the pipeline. The amount of natural gas transported between the supply and demand sides is determined by the metering data of the flow measurement device installed on the gas pipeline. This flow measurement data is also an important basic data for measuring the unit production technology level. Accurately measuring natural gas flow is a key link for enterprises to conduct economic analysis and reduce operating costs. It directly affects the economic benefits of enterprises and has attracted much attention from both supply and demand sides. As the world's energy supply and demand become increasingly tense, people are very concerned about and seek a high-precision and highly adaptable flow meter to measure natural gas flow.

1 Differential pressure flowmeter Although differential pressure gas flowmeter has become increasingly mature, its types are still extremely numerous and its performance varies. Before the 1950s, differential pressure flowmeter was the most popular natural gas flowmeter, accounting for about 50%-60% of the total flowmeter, and the sales volume in my country was about 40%. It is an instrument that measures flow based on the differential pressure generated by the flow detection component installed in the pipeline, the known fluid conditions and the geometric dimensions of the detection component and the pipeline. It has a long history of application, rich and mature practical experience, perfect standards and specifications (except for the average velocity tube), and a full range of varieties and specifications. In addition, it also has the advantages of simple structure, durability, strong versatility, low cost, and standard throttling components that do not require actual fluid calibration.

The differential pressure flowmeter also has many disadvantages. Its on-site installation conditions are high and require a long straight pipe section. The accuracy of the whole set of flowmeters is also determined by the accuracy of the differential pressure transmitter and the flow display. It is a very strict instrument from design, manufacturing to installation and use. Any error in any link will cause a large error. In addition, the output signal of the differential pressure flowmeter is in a square relationship with the flow rate. It is a nonlinear instrument with a narrow range. The large pressure loss is also one of its weaknesses. In response to the above shortcomings, domestic manufacturers have introduced a series of improvement measures in recent years, such as integrated differential pressure flowmeters, fixed value throttling parts, interchangeable orifice throttling devices, standard nozzles, etc., and the two components of differential pressure flowmeters since the 1980s: differential pressure transmitters and flow displays have made outstanding progress. It is estimated that this type of flowmeter will continue to occupy an important position in the future. Channel flowmeters, V-cone flowmeters, and swirl flowmeters are three representative new flowmeters, all of which belong to differential pressure flowmeters.

2 Classification of differential pressure flowmeters There are many types of differential pressure flowmeters. According to different classification methods, they can be divided into three categories and 20 types. For specific classification, see Table 1. Table 1 Classification of differential pressure flowmeters Classification principle Classification type Throttling type; Dynamic pressure head type; Classification by the principle of generating differential pressure (3) Hydraulic resistance type (4) Centrifugal type; Dynamic pressure gain type Jet type Standard orifice plate; Standard nozzle; Classic Venturi tube; Venturi nozzle Conical inlet orifice plate; 1/4 circular orifice plate; Segmented orifice plate; Eccentric orifice plate; Classification by structural form (9) Wedge-shaped orifice plate; Integral (built-in) orifice plate; Linear orifice plate; Annular orifice plate; Doyle tube; Rollos tube; Elbow tube; Interchangeable orifice plate throttling device; Critical flow throttling device Hu Haiyan (1979-), female, from Zhanhua, Shandong, engineer, graduated from China University of Petroleum, engaged in measurement technology.

Table continued Classification principles Classification type Standard throttling device; Low Reynolds number throttling device; Dirty flow throttling device; Classification by purpose Low pressure loss throttling device; Small diameter throttling device; Wide range throttling device; Critical flow throttling device 3 Selection of differential pressure flowmeter With the development of science and technology, many new types of flowmeters are constantly emerging. However, due to the wide range of natural gas flow measurement technology itself, different users have different requirements for flowmeters, that is to say, objectively, multiple flowmeters coexist to meet the requirements of different levels and occasions. Different types of flowmeters have different instrument characteristics and fluid characteristics, and the required installation conditions and use conditions are different. Each flowmeter has its scope of application and limitations. This requires us to consider various factors at the same time when selecting. When selecting an instrument, we must be familiar with the object to be measured, instrument performance, measurement environment and other aspects.

3.1 The purpose of using the flowmeter of the measured object should be very clear. That is, it is necessary to know whether the measured flow is volume flow or mass flow; whether the measured flow is instantaneous flow or cumulative flow. However, due to differences in structure and principle, flowmeters are often only suitable for measuring one flow or measuring a certain flow at most. Measuring other flows requires adding conversion equipment or reducing accuracy. For example, the vortex flowmeter relies on counting or pulse number to obtain the cumulative flow value, which is suitable for measuring the total amount. If it is used to measure the instantaneous flow, a timing device and a corresponding calculation circuit must be added to convert the instantaneous value. During the conversion process, its accuracy will inevitably be reduced due to factors such as effective digits.

First of all, the properties of the fluid should be considered. For example, for incompressible fluids, volume flow and mass flow can be converted, that is, the velocity and density related variables can be used to obtain the velocity and density when they are constants).

There are two situations for natural gas measurement: one is the measurement of unclean natural gas before processing, and the other is the measurement of clean natural gas after processing. Unclean natural gas often contains liquids, solid particles and other precipitates that may contaminate the gas flow meter, causing trouble for measurement. Clean natural gas after processing is sometimes unclean during the gathering and transportation process due to poor processing technology and unclean gathering and transportation pipelines.

3.2 The installation conditions of the measurement environment often depend on the requirements of the on-site space and orientation. For example, the vortex flowmeter often requires the installation direction of the sensor and the flow direction to be horizontal, vertical and horizontal respectively, and due to the influence of factors such as flow velocity distortion, a certain length of straight pipe needs to be added upstream or downstream to eliminate interference.

When the flow meter is in use, the main environmental factor that affects it is electromagnetic interference. For example, low-frequency electromagnetic interference, that is, when it is used on site, comes from the AC power supply (220V50HZ. Because during installation, in order to standardize the wiring, it is often through a cable tray or protective tube, so that the AC power supply and the signal line may be parallelly routed together, and the interference signal will be coupled to the signal output line. It can be seen that increasing the distance between the power line and the signal line, reducing the distribution and reasonably selecting the signal line grounding point can avoid the influence of interference voltage on the flow meter.

3.3 Instrument Performance In gas flow measurement, temperature and pressure correction is one of the important means to improve the gas flow measurement accuracy.

Whether there is temperature and pressure correction is also one of the issues that need to be considered when selecting. The deviation caused by not making pressure correction is definitely negative, and the greater the actual working pressure, the greater the deviation. Without temperature correction, when the ambient temperature is higher than 20*C, the deviation caused is positive; when the ambient temperature is lower than 20*C, the deviation caused is negative, so it is very important to make temperature and pressure corrections.

What is the overall measurement accuracy requirement? Is it used at a specific flow rate or within a certain measurement range? Within what measurement range can the above accuracy be maintained? How long can the accuracy of the selected instrument be maintained? Is it easy to recalibrate? Is it necessary to check the instrument accuracy on site online? These issues must be carefully considered.